This invention relates to dynamoelectric machines and more particularly to drum-type homopolar machines.
Drum-type homopolar dynamoelectric machines include a stationary excitation system and a rotating drum composed of a combination of ferromagnetic and highly conductive materials configured such that a direct current output voltage is produced along the axial length of the drum. These machines incorporated a set of current collection members at either axial end of the rotor which carries full load current. Homopolar dynamoelectric machines may operate as either a motor or generator and are particularly suited to transfer energy in short, high-current pulses to a storage inductor and a final load consisting of a resistive-inductive system, as for example, an electromagnetic launcher. The rotor of drum-type homopolar machines may include a cylindrical shell of a highly conductive, non-ferromagnetic material which generates and supports the full load current. This member is bonded or shrunk onto a ferromagnetic inner cylindrical core which serves as the main rotor body and is directly attached to a drive or input shaft. Both components of the rotor are, preferably, homogeneous materials without segmentation or any combination of axial or circumferential grooves. It is imperative that the rotor surface on the two axial ends be smooth since this zone is used exclusively for current collection with, for example, solid metal-graphite brushes. The machine's internal electromotive force is confined to an axial zone along the center of the rotor between the two outer current collection zones.
Drum-type homopolar machines may be classified as truncated drum or full drum types according to the relative rotor active length. The excitation system includes a stator having a main pole piece which is used to confine the magnetic flux to a zone of the rotor which is directly in line radially with the main pole piece. It is desirable that the total machine flux should only cut the rotor surface at a location which is separated from the current collector zone. In practical machines, with significant iron-iron air gaps, magnetic saturation of the core material or poles and conventional pole tip geometries, an amount of leakage flux will typically pass from the main pole side across the air gap at a non-radial angle and enter the rotor magnetic circuit through the current collection zone. It is this leakage flux which causes an additional voltage to be generated in the rotor current collection zone under the brushes. The particular construction of the rotor shell which includes a continuous homogeneous cylinder in conjunction with the use of a relatively long brush collector at each end, creates additional induced electromotive force due to the leakage flux that results in large continuous circulating current in closed, short circuiting loops composed of the rotor conductor and each brush box at every point along the circumference.
This has been a recurring problem in conventional drum-type homopolar generators and alternative correction means have been shown in the prior art to reduce the magnitude of the brush to rotor circulating currents. Some conventional methods of reducing circulating current are as follows:
1. Increasing the rotor diameter of the machine with a significant decrease in rotor collector lengths; PA1 2. Keeping the collector length as small as possible by increasing the collector current density; PA1 3. Attaching each brush or module to separate load circuits as well as extending the lead length of the individual brush modules to a maximum length so as to increase the effective resistance of the circulating current path; and PA1 4. Adding large magnetic dead zones or air spaces between the excitation stator field coil/pole shoe and the start of the current collection zone.
However, in the interest of building lightweight and extremely compact designs with a low moment of inertia, all of the above conventional methods have proven to be cumbersome and unable to meet minimum weight criteria. In assessing any conventional homopolar generator, a significant percentage of the total field magnetomotive force or ampere turns directly contributes to magnetizing the rotor in undersirable zones and even in locations such as the bearing supports.